A. Technical Field
The present invention relates generally to transmission line technology, and more particularly, to a transmission line that connects wirebond packages of semiconductor devices and boards.
B. Background of the Invention
Chips used in electronic circuits typically need to be connected to a board on which an electrical system is located. The electrical inter-connection from the chip package to a board can be done by wirebonding techniques. This generally involves using thin metal wires such as gold to connect an IC chip on one side and to outer electrodes on the board.
Wirebonded electrical packages oftentimes behave as transmission lines, especially when data is being communicated on the wirebond at high speeds. In particular, bondwires have associated inductive characteristics that may degrade the performance of the transmission lines. The degradation in performance may be caused due to poor impedance matching, which results in poor return loss characteristics of the lines.
FIG. 1 illustrates an exemplary wirebond package that provides a connection(s) between an IC and a board. The package 100 comprises a driver 110 that drives a load 120. The interconnection between the driver stage and the load 120 occurs through a bondwire 135 and a package trace 140. The driver 110 is modeled to have driver impedance represented by a resistor 115, and the driver capacitance represented by capacitor 130. As previously mentioned the bondwire 135 may act as a transmission line and causes the transmission line to have an inductive impedance. This leads inductive impedance may result in an impedance mismatch between the IC and the board.
The bondwire inductance in a wirebond package may be matched by placing a capacitor in shunt with the inductive bondwire. The shunt capacitor acts by resonating out the inductive impedance of the bondwire. Although this technique is effective in impedance matching, the use of a discrete capacitive component also has drawbacks.
The additional discrete capacitor requires space and may adversely affect the package. The cost of the package implementation may increase due to the addition of the discrete capacitor and may also require modifying the manufacturing process of the IC and/or package. Alternatively, if the capacitor used is integrated on the silicon, then space is consumed on the silicon increases. Electromagnetic compatibility may also be affected due to electrostatic discharge (ESD) caused due to the integrated capacitor.
The discrete capacitor may only properly operate and provide resonation within a narrow band of frequencies. If the package is used over a broader spectrum of frequencies, the characteristics of the capacitor may significantly change and improperly function over certain frequencies. As a result, the actual return loss of the circuit or package varies and may not satisfy various requirements, such as a broadband requirement. Accordingly, the actual applications of an IC package may be limited by its impedance characteristics.
What is needed is an apparatus and method that address the above-described concerns.